Image forming apparatus

Abstract

An image forming apparatus starts to change a curl correcting amount until when an upstream end in a sheet conveying direction of a preceding sheet passes through a curl correcting unit and finishes changing the curl correcting amount until when a succeeding sheet that follows the preceding sheet reaches the curl correcting unit in a case when the preceding sheet and the succeeding sheet are both guided in the curl correcting unit through a conveying path and when the curl correcting amount is changed between curl correcting amounts of the preceding sheet and the succeeding sheet.

What is claimed is: 
     
       1. An image forming apparatus comprising:
 an image forming unit configured to form a toner image on a sheet; 
 a fixing unit configured to fix the toner image on the sheet; 
 a curl correcting unit configured to correct a curl of the sheet that has passed through the fixing unit and to vary a curl correcting amount for correcting the curl of the sheet; 
 a control unit configured to control the curl correcting amount by the curl correcting unit to start changing the curl correcting amount from a first curl correcting amount for correcting a curl of a preceding sheet to a second curl correcting amount for correcting a curl of a succeeding sheet that follows the preceding sheet before an upstream end in the sheet conveying direction of the preceding sheet passes through the curl correcting unit, to finish changing the curl correcting amount before the succeeding sheet reaches the curl correcting unit, and to set the curl correcting amount a the second curl correcting amount when a downstream end in the sheet conveying direction of the succeeding sheet passes through the curl correcting unit. 
 
     
     
       2. The image forming apparatus according to  claim 1 , further comprising a reverse conveying path configured to guide the sheet from the fixing unit by reversing the downstream end in the sheet conveying direction and the upstream end in the sheet conveying direction of the sheet such that the downstream end in the sheet conveying direction at the fixing unit is changed to the upstream end in the sheet conveying direction of the sheet at the curl correcting unit;
 wherein the control unit controls the curl correcting unit to start changing the curl correcting amount after an upstream end in the sheet conveying direction of the preceding sheet passes through the curl correcting unit, to finish changing the curl correcting amount after a downstream end in the sheet conveying direction reaches the curl correcting unit and before an upstream end in the sheet conveying direction of the succeeding sheet reaches the curl correcting unit and to keep the curl correcting amount at the second curl correcting amount until at least the upstream end in the sheet conveying direction of the succeeding sheet passed through the curl correcting unit in a case when the preceding sheet and the succeeding sheet are both guided in the curl correcting portion through the reverse conveying path. 
 
     
     
       3. The image forming apparatus according to  claim 1 , wherein the curl correcting unit comprises:
 a pair of curl correcting rollers composed of rollers having different hardness and forming a curved nip portion by press-contacting with each other; and 
 a curved amount modifying portion configured to modify a curved amount of the curved nip portion by changing press-contact pressure of the pair of curl correcting rollers; 
 wherein the curl correcting unit corrects the curl of the sheet by passing the sheet through the curved nip portion of the pair of curl correcting rollers, and 
 wherein the control unit controls the curved amount modifying portion to modify the curved amount of the curved nip portion corresponding to a type of a sheet. 
 
     
     
       4. The image forming apparatus according to  claim 2 , wherein the curl correcting unit comprises:
 a pair of curl correcting rollers composed of rollers having different hardness and forming a curved nip portion by press-contacting with each other; and 
 a curved amount modifying portion configured to modify a curved amount of the curved nip portion by changing press-contact pressure of the pair of curl correcting rollers; 
 wherein the curl correcting unit corrects the curl of the sheet by passing the sheet through the curved nip portion of the pair of curl correcting rollers, and 
 wherein the control unit controls the curved amount modifying portion to modify the curved amount of the curved nip portion corresponding to a type of a sheet. 
 
     
     
       5. The image forming apparatus according to  claim 3 , wherein the curl correcting unit includes first and second curl correcting portions disposed along the sheet conveying direction and each having the pair of curl correcting rollers forming the curved nip portion, and
 wherein the pair of curl correcting rollers of the second curl correcting portion is configured to curve in a different direction from a curving direction of the pair of curl correcting rollers of the first curl correcting portion. 
 
     
     
       6. The image forming apparatus according to  claim 1 , further comprising an input portion configured to input types of sheets;
 wherein the control unit controls the curl correcting unit to change the curl correcting amount from the curl correcting amount corresponding to a type of a preceding sheet to the curl correcting amount corresponding to a type of a succeeding sheet when the control unit judges that the types of the preceding sheet and of the succeeding sheet are different based on sheet information input through the input portion. 
 
     
     
       7. An image forming apparatus comprising:
 an image forming unit configured to form a toner image on a sheet; 
 a fixing unit configured to fix the toner image on the sheet; 
 a curl correcting unit configured to correct a curl of the sheet that has passed through the fixing unit and to vary a curl correcting amount for correcting the curl of the sheet; and 
 a control unit configured to control the curl correcting amount by the curl correcting unit to start changing the curl correcting amount from a first curl correcting amount for correcting a curl of a preceding sheet to a second curl correcting amount for correcting a curl of a succeeding sheet that follows the preceding sheet after an upstream end in the sheet conveying direction of the preceding sheet passes through the curl correcting unit, to finish changing the curl correcting amount of the curl after a downstream end in the sheet conveying direction of the succeeding sheet reaches the curl correcting unit and before an upstream end in the sheet conveying direction of the succeeding sheet reaches the curl correcting unit, and to keep the curl correcting amount at the second curl correcting amount until at least the upstream end in the sheet conveying direction of the succeeding sheet passing through the curl correcting unit. 
 
     
     
       8. The image forming apparatus according to  claim 7 , wherein the curl correcting unit comprises:
 a pair of curl correcting rollers composed of rollers having different hardness and forming a curved nip portion by press-contacting with each other; and 
 a curved amount modifying portion configured to modify a curved amount of the curved nip portion by changing press-contact pressure of the pair of curl correcting rollers; 
 wherein the curl correcting unit corrects the curl of the sheet by passing the sheet through the curved nip portion of the pair of curl correcting rollers, and 
 wherein the control unit controls the curved amount modifying portion to modify the curved amount of the curved nip portion corresponding to a type of a sheet. 
 
     
     
       9. The image forming apparatus according to  claim 8 , wherein the curl correcting unit includes first and second curl correcting portions disposed along the sheet conveying direction and each having the pair of curl correcting rollers forming the curved nip portion, and
 wherein the pair of curl correcting rollers of the second curl correcting portion is configured to curve in a different direction from a curving direction of the pair of curl correcting rollers of the first curl correcting portion. 
 
     
     
       10. The image forming apparatus according to  claim 7 , further comprising an input portion configured to input types of sheets;
 wherein the control unit controls the curl correcting unit so as to change the curl correcting amount from the curl correcting amount corresponding to a type of a preceding sheet to the curl correcting amount corresponding to a type of a succeeding sheet when the control unit judges that the types of the preceding sheet and of the succeeding sheet are different from sheet information input through the input portion. 
 
     
     
       11. An image forming apparatus, comprising:
 an image forming unit configured to form a toner image on a sheet; 
 a fixing unit configured to fix the toner image on the sheet; 
 a sheet processing unit configured to process the sheet on which the toner image has been fixed by the fixing unit; 
 a pair of curl correcting rollers disposed along a sheet conveying path between the fixing unit and the sheet processing unit and composed of an elastic roller and a metallic roller whose hardness is higher than hardness of the elastic roller and which forms a nip by press-contacting the elastic roller; 
 a control motor configured to change an amount of bite of the metallic roller into the elastic roller by changing relative position of the elastic roller and the metallic roller; 
 a conveying path configured to guide the sheet from the fixing unit to the curl correcting rollers while keeping a downstream end in a sheet conveying direction of the sheet at the fixing unit as the downstream end in the sheet conveying direction at the pair of curl correcting rollers; 
 a reverse conveying path configured to guide the sheet from the fixing unit by reversing the downstream end in the sheet conveying direction and an upstream end in the sheet conveying direction of the sheet such that the downstream end in the sheet conveying direction at the fixing unit is reversed to the upstream end in the sheet conveying direction of the sheet at the pair of curl correcting rollers; and 
 a control unit configured to control the amount of bite by controlling the control motor corresponding to a type of the sheet to be conveyed and of executing a first mode when the sheet is conveyed to the pair of curl correcting rollers through the conveying path and a second mode when the sheet is conveyed to the pair of curl correcting rollers through the reverse conveying path, the control starting to switch the amount of bite in the middle of a preceding sheet passing through the nip, finishing to switch the amount of bite before a succeeding sheet that follows the preceding sheet reaches the nip, and setting the amount of bite at a changed amount of bite when the succeeding sheet reaches the nip in the first mode, and starting to switch the amount of bite in response to the rear edge of the preceding sheet passing through the nip in the second mode.